Sewage treatment plants generally consist of a primary settling tank wherein large particles are removed by screening or are settled through use of a flocculant such as aluminum chloride. The partially clarified waste water containing fine particles and colloidal materials which are generally organic in nature is transferred to a secondary waste water treatment tank for aeration of the waste water. A source of microorganisms is introduced into the secondary tank to digest the organic material contained in the water transferred from the primary treatment tank. After passage through the secondary treatment tank the water with digested waste material is transferred to a settling tank where the digested organic material settles to the bottom of the tank as sludge and clarified water passes from the top of the tank. The clarified water generally had 30 ppm BOD or less. The clarified water may then be subjected to tertiary treatment by chlorination or other means. The sludge from the settling tank generally is divided into two streams, an outlet sludge take-off stream and a recycle sludge stream. The recycle sludge stream is reintroduced into the secondary waste water treatment tank for aeration and further reduction in organic content and to supply microorganisms for the secondary waste water treatment tank.
It is known in the sewage treatment industry to introduce oxygen into the water stream from the primary treatment tank prior to introduction into the secondary waste water treatment tank. In general, oxygen is injected at a level sufficient to provide a saturated level of oxygen of about 20 to 30 ppm. The oxygenated water stream from the primary treatment tank is then introduced into the secondary waste water treatment tank. The recycled sludge stream has substantially no oxygen content and the microorganisms are in a dormant state. The combined recycled sludge stream and the oxygenated waste water stream from the primary treatment tank results in producing a level of oxygen in the secondary waste water stream of less than about 2.5 ppm after a period of stabilization. This is the maximum level of oxygen that can be tolerated by the microorganisms during aeration and digestion of the waste in the secondary waste water treatment tank.
A primary difficulty with oxygenation of the waste water stream from the primary treatment tank is that a substantial amount of the residence time of the organic waste in the secondary waste water treatment tank is consumed by activating the microorganisms contained in the recycle sludge stream. The present invention is directed to a method for activation of such microorganisms by a direct injection of oxygen into the recycle sludge stream.